Optical sensors employ optical elements that are used in the collection of optical radiation and direct such collected optical radiation to an optical sensor element for measurement of sensor specific characteristics based on that collection. To collect such incident optical radiation, optical sensors typically use an optical window or lens as the initial optical element, although other optical elements may be incorporated between the window and the sensor element. A window generally acts as a barrier to protect the following optics from adverse environments, such as excess temperature or pressure. The optical surfaces of the window are flat and may be parallel, or tilted at a slight angle relative to each other to form an optical wedge. A window with parallel optical surfaces does not change the divergence angle of the incident optical radiation. An optical wedge will slightly change the direction of incidence and the divergence angle of the incident optical radiation. Additionally, one or both of the optical surfaces of a lens are curved and act to change the divergence angle of the incident optical radiation to effectively direct the incident radiation onto the active area of the sensor.
Optical elements are most commonly used for collecting axial optical radiation, i.e., optical radiation incident over a range of angles near the axis of the surface normal of the initial optical element. In many optical sensor applications, this angular extent is limited by the acceptance angle of conventional optical elements. Such optical elements are typically limited to an acceptance angle of about 30° (half cone angle). In optical terms this represents a numerical aperture of about 0.5, defined as the sine of the half cone angle, or an f number of approximately 1, where the diameter of the optical element equals its focal length. In some sensor applications, however, it is desirable to have optical elements that are able to detect non-axial, i.e. off-axis radiation at large angles of incidence, for example from 60 to 80°. This is difficult using conventional optical elements.
When optical radiation is incident on an optical window element at large angles of incidence, although the angle of incidence is reduced inside the window element, as prescribed by Snell's Law, the exit angle of the optical radiation is the same as the incident angle. This means that sensors placed behind a planar window must be oriented so that the sensor element surface normal is parallel to the optical axis of the incident optical radiation. And, in practical sensor applications, such placements lead to unacceptably large packaging volumes, especially when multiple sensors are employed.
Another problem with the use of conventional optical elements at large angles of incidence is Fresnel reflection. As is well-known, Fresnel reflections cause a loss of transmitted radiation at transmissive surfaces due to a reflection from the same surface that increases in magnitude as the angle of incidence increases. For materials of higher refractive index, such as semiconductor materials, these Fresnel reflections can result in sizeable losses of optical radiation. As a result of the foregoing, optical sensor designers are limited in their ability to design effective off-axis optical radiation sensors.
Many types of optical radiation detector have small surface areas, and practical sensor designs usually require small packaging volumes. In conventional optical radiation configurations, this requires the use of a lens with a short focal length. Such lenses are also subject to Fresnel losses. Additionally, they introduce focusing errors such as large optical aberrations for optical radiation at high angles of incidence. These aberrations increase as the f number of the lens is decreased and as the refractive index is increased. This is especially true of semiconductor materials such as silicon or germanium that have extremely high refractive indices compared to air. These focusing errors can significantly reduce sensor performance and impact the effectiveness of the overall system to perform its intended function.
Large lenses with short focal lengths have an almost hemispherical shape. This makes them heavy and difficult to fabricate. Furthermore, in aircraft applications, weight and airflow disturbance considerations require a lens that is similar in profile to the immediate surface contour of the external vehicle. To address this, lenses with a faceted structure, known historically as Fresnel lenses have been used. If such facets are configured to be linear, a cylindrical Fresnel lens is formed that acts in a similar fashion to a conventional cylindrical lens. A collimated beam incident along the optical axis of the lens is bought to a line focus. If, instead, the facets are configured in a circular pattern, a spherical Fresnel lens is formed that acts in a similar fashion to a conventional spherical lens. A collimated beam incident along the optical axis of the lens is bought to a point focus.
These generally flat lenses collect substantially axial radiation, i.e., radiation over an angular sensitivity range that includes a surface normal, and use that collected radiation for imaging or illumination purposes. Some have recently proposed Fresnel lenses that include binary optical structures formed of discrete steps, which may be formed via photomasking techniques. Yet, these Fresnel lens share operation and design with the other classes of lens. They are designed for collection of axial or near axial radiation, not off-axis radiation collection. Other grating structures have been proposed, including diffraction gratings, for example, and large-scale plastic grating structures. But such structures operate by refraction and exhibit some of the alignment and distortion phenomena discussed above, especially for off-axis radiation.
Others have suggested the use of discrete microprisms for side illumination in instrument displays. But, the radiation in such structures is deflected by a complex arrangement of prisms that makes uniform collection and transmission of light over a sensing area very difficult. Further, the microprism structures are not periodic structures.
There have also been suggestions for arrays of microlenses, in applications such as micro-scale chemical analysis, fiber optic coupling and stereoscopic displays. These are imaging-type optical elements that operate by refraction and, as a result, can exhibit losses and distortions compared to the ideal, desired operation. Non-imaging microstructures have also been suggested for light concentration onto detector arrays using reflective devices, but again the devices are designed to collect light incident substantially on axis, not off axis. Plus, these structures do not focus light and may direct light back out of the collection aperture at large angles of incidence.
Combinations of diffraction gratings and lenses have also been proposed to correct for the chromatic aberration of conventional lens in applications such as photography, image projection and telescopes. But there is no capability to collect off-axis radiation, as they are limited to axial collection.
None of the various known optical elements are able to effectively collect off-axis, especially near grazing incidence, light in a low-loss system. Either from limitations on the angles of the light that can be collected or from the losses that affect that light once collected (e.g., Fresnel reflections), these optical elements present design limitations to sensor manufacturers.